Here is the ultimative Speedbike Tip:
1.Run to a Speedbike
2. Hit "E" to take control of it
3. Close your eyes
4. Stay cool!
5. Press "W" or whatever your acceleration button is
6. Now you should be able to "feel" the rocks and trees" in front of you
7. If you still crash you never will be a Jedi !
Hope this helps